Diagnostic tests frequently require separation of a patient's whole blood sample into components, especially cellular portions from non-cellular portions e.g. serum or plasma from cells. For instance, plasma is obtained from anticoagulated blood and still contains all of the coagulation proteins, whereas serum is obtained from clotted blood with the proteins being retained with the clot and red blood cells. Samples of whole blood are typically collected by venipuncture through a special cannula or needle attached to a syringe or an evacuated collection tube. The sample of blood that is to be separated into components is typically drawn, using a needle, through a penetrable self-sealing elastomeric closure or other stopper into an evacuated tube. Separation is then accomplished e.g. by rotation of the tube in a centrifuge.
The centrifuge may be a conventional centrifuge e.g. a swinging bucket or a fixed angle centrifuge, as the different components of the whole blood have different densities, as described in U.S. Pat. No. 4,152,269 of A. L. Babson. Alternatively, the centrifuge may be an axial centrifuge i.e. a centrifuge in which separation of the phases is achieved by rotation of the tube about its longitudinal axis i.e. axial rotation, as described in U.S. Pat. No. 4,828,716 of J. A. McEwen et al. In the latter process, the blood sample is introduced to the tube through a cap assembly that consists of a piercable closure and a separator that has a one-way valve. The tube is then rotated about its longitudinal axis; the heavier cellular phase lines the tube wall and thereby separates from the lighter non-cellular (plasma or serum) phase. Once separation has been achieved, an axial probe penetrates the piercable closure, detaches the separator from the closure and forces the separator down the tube. The axially-located non-cellular phase passes through the separator. An optical sensor is utilized to detect when the cellular phase begins passing into the separator, and to stop movement of the separator. Thus, the two phases are physically separated.
A known separator that is believed to be effective in maintaining the phases in a physically separated form is disclosed in copending U.S. patent application of G. A. Adams and R. P. Luoma, No. 07/877,496, filed May 01, 1992, now U.S. Pat. No. 5,282,198, A double ended tube for use in an axial centrifuge is disclosed in copending U.S. patent application of R. P. Luoma, No. 07/887,497, filed May 01,1992 now U.S. Pat. No. 5,271,852.
In routine operation of an axial separation process, it is important that the process be capable of being controlled in a consistent manner, in addition to the need to minimize and preferably avoid cross-contamination of samples.